Pillar page (or post as you put it) is a loose term to descirbe a long, comprehensvie peiece of content with valuable and consistent internal linking from other pages, usually blog posts.

Add the cluster posts to the pillar page by means of RSS shouldn't do any harm, but it might not have much a positive impact in terms of SEO. The generally recommended strategy is to hyperlink from the text of those cluster peices of content to the pillar page using consistent language.